This amended P30 ACISR application is submitted by the Department of Sociomedical Sciences at Columbia's Mailman School of Public Health in response to the IRG Summary Statement of April 15, 2004. We propose to create a multi-disciplinary research center focused on the prevention of chronic homelessness among people with severe mental illness. Using a conceptual framework grounded in prevention theory, the application presents a research agenda targeted at two overall objectives;the first is the prevention of chronic homelessness among people with mental illness. We will develop preventive interventions for this high-risk group to enable individuals living with mental illness to retain safe, adequate, and affordable housing throughout the lifespan, enhancing their ability to be included fully in the life of the community. The second objective is to address the prevention of chronic homelessness at the population level, in order to target those who are vulnerable to homelessness and might not yet be identified as mentally ill, and therefore cannot be reached by high-risk interventions. The proposed Columbia Center for Homelessness Prevention Studies has three Cores: (1) the Operations Core, to provide leadership to both stimulate and manage innovative research that meets the highest scientific standards, ethical integrity, and efficient fiscal operations;(2) the Principal Research Core, to develop an agenda of research focused on interventions to prevent chronic homelessness among people with severe mental illness, the highest risk group, with the long range objective of addressing homeless prevention at the population level;and (3) the Methods Core, to provide a centralized multi-disciplinary resource for the development and execution of pre-intervention and intervention studies needed to carry out a research agenda on prevention of chronic homelessness. Operations Core